When a negative working light-sensitive lithographic printing plate comprising a hydrophilic support with a negative working light-sensitive resin composition provided thereon in a thin layer form is exposed to light through a negative transparency, a photo-chemical reaction occurs at exposed areas. The reaction causes the light-sensitive resin composition to form a net work structure or cross-linked structure, or to change the adhesive force of the light-sensitive resin composition with respect to the support. This accelerates insolubilization of the light-sensitive layer in a developer. When the light-sensitive layer is subjected to the developer, unexposed parts of the light-sensitive layer are removed, resulting in the formation of a positive image pattern on the support.
In such negative working light-sensitive printing plates, therefore, as the exposure time is increased or lengthened, the net work structure of the light-sensitive resin composition, or the adhesive force of the light-sensitive resin composition to the support is enhanced, as a result of which the stability of the image pattern at the changes of developing conditions, e.g., a change of developing time, is increased and the strength of the thus-formed image is also increased, resulting in a production of a lithographic printing plate having longer press life.
However, when the exposure time is increased to obtain such printing plates having longer press life, the image pattern tends to lose sharpness or cause so-called dot-gaining under the influences of scattering or reflection of light. This tendency becomes pronounced particularly when supports readily scattering light, such as a grained aluminum support, and a paper support are used. The tendency becomes even more pronounced when such grained aluminum supports are subjected to an anodic treatment. In order to improve characteristics such as printing performance, the thickness of the anodized layer on the support is sometimes increased. This makes the image pattern lose sharpness and in the case of half tone dot images, the resulting gaining tendency of the dot size, deteriorates tone reproduction especially in shadow areas. Thus, the entire dot image becomes dark and it becomes impossible to obtain images having the desired tone.
Various methods of preventing the half tone dot gaining in negative working light-sensitive lithographic printing plates are known, including a method in which the light-sensitive layer is made as thin as possible; a method in which the light-sensitive layer is designed so that the absorbance thereof does not drop too much during exposure, for example, dye is incorporated into the light-sensitive layer; and a method in which the support is subjected to a coloring treatment or is undercoated with dye to prevent halation of light from the support.
In connection with the method of preventing the halation of light from the support a paper master as described in British Pat. No. 1,129,407 in which a yellow colored support is used, or a light-sensitive lithographic printing plate as described in U.S. Pat. No. 3,891,516 in which a steel-gray anodized aluminum plate is used. Further, lignt-sensitive lithographic printing plate as described in U.S. Pat. No. 3,458,311 in which a support is undercoated with yellow dye, and a light-sensitive layer is provided thereon, and so forth are known. The method of decreasing light reflectivity of the support surface has many disadvantages, for example, the light-sensitive layer has a poor adhesivity to the support, the lithographic printing plate is liable to produce scum, and images obtained are indistinct due to dark color of the support.
The incorporation of dye into the light-sensitive layer is a method which has long been employed for optically uneven light-sensitive layers of silver salt emulsions in which the irradiation of light by silver halide particles occurs strongly. In accordance with this method, the resolving power and image sharpness are improved by adding dye to the light-sensitive layer. In the case of non-silver salt light-sensitive materials, an attempt to add certain ultraviolet absorbers to the light-sensitive layer so as to increase gradation and resolving power is disclosed in U.S. Pat. No. 4,144,067.
The sensitivity of the light-sensitive layer, however, is lowered and, therefore, the material is not proper for a practical use.
In the method of preventing the half tone dot gaining by reducing the thickness of the light-sensitive layer, the material having a thin light-sensitive layer is not only improved in an image reproductivity to the extent expected but also shows lower press life. In fact, it is impossible in a practical use to decrease the coating amount to 100 mg/m.sup.2 or less. Therefore, the method to make a thin light-sensitive layer has a limitation in its thickness of the light-sensitive layer.
As a result of our investigation of various dyes which do not lower the sensitivity of a light-sensitive layer when added to the light-sensitive layer and which reduce the gaining of half tone dots and improve the image sharpness, the inventors have attained the present invention.
Thus, the object of the present invention is to provide a negative-working light-sensitive composition reducing the gaining of half tone dots and to improve image sharpness without lowering the light sensitivity and to provide a negative working light-sensitive lithographic printing plate having the layer comprising the above-described composition on a support.